Liquid toner compositions for use in developing latent electrostatic images are well-known in the art. However, in order for a toner to be suitable for use, particularly in a gap transfer xeroprinting process, it requires the following properties:
(a) the ability to use standard pigments in the toner formulation;
(b) the toners used for multicolor printing must be transparent. This is achieved by making a fine homogeneous dispersion of the pigment within a dispersed phase binder. All of the toner components must be compatible in order that they can be fused into a clear, transparent film;
(c) the toner needs a relatively large particle size in order to reduce surface area and Van der Waals forces in order to achieve high transfer efficiency;
(d) the toner must be easily dispersed after it settles, so as to eliminate any gelling problems in the machine in which it is used;
(e) the toner must image with excellent resolution, good solid densities, and no background on the electrostatically imageable surface, and it must maintain these properties during the transfer steps;
(f) the toner must have good adhesion to paper when fused; and
(g) the toner system should behave relatively independently of the pigments used so that different color toners can be mixed together to produce a desired shade.
Known toner materials have only fulfilled the above-discussed requirements to a limited extent. Useful liquid toners comprise a resin and nonpolar liquid. Generally, a suitable colorant is present, such as a dye or pigment. The color toner particles are dispersed in the nonpolar liquid which generally has a high volume resistivity, a low dielectric constant, and a high vapor pressure. These toners are generally prepared by forming a dispersion of a resin, nonpolar liquid, and colorant and then milling the dispersion with more nonpolar liquids and other desired additives. This preparation is easy but is very difficult to design properly. The performance of the toner is very pigment dependent, and each color would need to be formulated separately. It is difficult to make a transparent toner using such methods.
A method of formulating a nonhazy or transparent toner is described in U.S. Pat. No. 4,507,377. The toner is made from a compatible blend of a polyester resin and a polyester plasticizer characterized in that it is substantially insoluble in the carrier liquid. The toner in the patent is self-fixing and not used in a transfer system.
A disadvantage of the toner system of said patent is that we have found that it is difficult to disperse pigments into polyester systems. Also, the polyesters tend to swell in a carrier liquid, such as in Isopar. Thus, the toner system of U.S. Pat. No. 4,507,377 does not meet the requirements, discussed above, for an acceptable toner system for transfer xeroprinting processes.
Another disadvantage of color liquid toners is that the toners do not claim to be usefully blendable to form distinct process colors. Because of the difference in electrophoretic mobility of each differently pigmented toner, a blend of two or more toners will selectively deplete as multiple images are made and the hue would continually change.
The toner of this invention behaves independently of the pigment used; in that each toner has identical electrophoretic mobility. They can be blended in the same manner as inks for spot color such as in the Pantone.TM. Color Matching System. Individual toners can be easily made using blends of pigment to give a special distinct hue.